Important raw materials necessary for the production of petroleum products such as ethylene and propylene are prepared by hydrocarbon steam cracking which is a major ingredient of natural gas or paraffin compounds such as naphtha and gas oil, at high temperature of at least 800° C. in the presence of water vapor.
To increase the yield of ethylene or propylene from hydrocarbon steam cracking, it has been largely attempted to increase hydrocarbon conversion rate or to increase olefin selectivity. However, there is a limitation in increasing hydrocarbon conversion rate or olefin selectivity depending only on steam cracking. So, alternatives have been proposed to increase olefin yield.
For example, using a catalyst for steam cracking was proposed as an alternative for the improvement of ethylene and propylene yields from hydrocarbon steam cracking. U.S. Pat. No. 3,644,557 describes the catalyst composed of magnesium oxide and zirconium oxide, U.S. Pat. No. 3,969,542 describes the catalyst containing calcium aluminate as a basic component, U.S. Pat. No. 4,111,793 describes the manganese oxide catalyst impregnated in zirconium oxide, European Patent Publication No. 0212320 describes the iron catalyst impregnated in magnesium oxide, and U.S. Pat. No. 5,600,051 describes the catalyst composed of barium oxide, alumina and silica. PCT No. 2004/105939 describes a method using the catalyst composed of potassium magnesium phosphate, silica and alumina. It has been known that these catalysts are acting as a heating medium during hydrocarbon steam cracking to increase olefin yield, but the increase of olefin yield is not satisfactory compared with when an inactive carrier is used.
Russian Patent No. 1,011,236 describes the potassium vanadate catalyst modified with boron oxide carried in an alumina carrier. However, using the potassium vanadate catalyst or alkali metal oxide experiences not only unsatisfactory improvement of olefin yield but also loss at high temperature for hydrocarbon decomposition. That is, the components of the catalysts have low melting points and thus exist possibly in liquid phase in the inside of a high-temperature cracking reactor and they are easily evaporated owing to the fast gas flow during the reaction, resulting in the loss of catalytic activity as reaction progresses.
U.S. Pat. No. 7,026,263 describes a method of using a hybrid catalyst composed of molybdenum oxide, alumina, silica, silicalite and zirconium oxide. The catalyst has an advantage of usability at low temperature reaction, but at the same time has a disadvantage of difficulty in direct addition to the actual production line because it is used at a very low hydrocarbon superficial velocity. In addition, thermostability of such catalyst becomes very low at the reaction temperature of 700˜800° C. or up, resulting in the loss of catalytic activity.
Most of the conventional cracking processes are performed at a high reaction temperature and with a high hydrocarbon superficial velocity and accompany the generation of a huge amount of coke. The generated coke has to be burned at a high temperature. To utilize a catalyst for a long time under such a severe condition, the catalyst has to be thermally/physically stable with less transformation. However, the above methods and skills are in question of thermal/physical stability.
To prevent economical loss in the process of hydrocarbon steam cracking and to avoid complicated production processes, an excellent catalyst which is capable of improving the yield of light olefin higher than an inactive carrier can do and is thermally/mechanically stable at high temperature is required.